There is an urgent demand in certain aerospace applications for a structural alloy having a yield strength of about 140,000 psi and a tensile strength of about 170,000 psi in heavy, thick-section forgings, along with good resistance to hydrogen embrittlement. Certain precipitation age hardenable iron base superalloys have been developed which are capable of this level of mechanical properties. The A286 alloy, which has a composition, by weight, of 13-15 percent chromium, 24-27 percent nickel, 1-2 percent molybdenum, 1.5-2.5 percent titanium, 0.1-0.5 percent vanadium, 0.003-0.010 percent boron, balance substantially iron, is one of these alloys.
Conventional processing for the A286 alloy includes final deformation cycles at 1800.degree. to 2000.degree. F., solution heat treatment at 1750.degree. to 1800.degree. F. for approximately to 1 hour, and precipitation heat treatment at about 1325.degree. F. for approximately 16 hours. This provides material with a typical yield strength of about 100,000 psi, and a typical tensile strength of about 160,000 psi.
U.S. Pat. No. 3,708,353, issued to Athey and developed by the Pratt & Whitney Division of United Technologies Corporation, describes a method for processing A286 material which provides improved properties. Rolling into sheet or strip in the temperature range of 1,550.degree. to 1,800.degree. F. produces material with extremely small grain size. Subsequent processing includes a stabilization operation at about 1,400.degree. F., followed by aging at about 1,300.degree. F. and provides a typical yield strength of about 160,000 psi and a typical tensile strength of about 175,000 psi.
It has been determined experimentally that applying this processing sequence to the same alloy in much thicker sections does not consistently generate the same level of mechanical properties. In the thin sheet or strip material utilized in U.S. Pat. No. 3,708,353, the rolling and cooling cycles are such that recrystallization of the material, which would dissipate the strain hardening, does not usually occur. For heavy, thick-section forgings, generally greater than about one inch in thickness, which retain heat longer than thinner material, similar thermomechanical processing of the same alloy generally results in recrystallization of the material and relief of the strain hardening imparted during the forging operation, not allowing a consequent improvement in the mechanical properties.
Thus, what is needed is a processing method for heavy, thick-section forgings of precipitation age hardenable iron base superalloys which produces a minimum yield strength of about 140,000 psi and a minimum tensile strength of about 170,000 psi.